Loads such as headlights and motors are built into vehicles, and such loads are connected to a vehicle-mounted power supply by electric wires. Electric wires degrade over time due to friction and the like, exposing the core wires inside, which may then come into contact with nearby conductive structures and cause short-circuits. Thermal fuses are typically inserted at appropriate locations of electric wires to prevent the electric wires or loads from being damaged by short-circuits.
However, when loads having high inrush current, such as headlamps, are repeatedly energized, thermal fuses tend to degrade, resulting in a shorter weld time. There is thus a problem in that a thermal fuse having a relatively high current capacity must be used, and as a result, an electric wire capable of withstanding a comparatively high current must be used as well.
As a method for solving such a problem, a protection device that estimates the temperature of an electric wire on the basis of a current value and, when the temperature of the electric wire reaches a limit temperature, cuts a conductive path from a vehicle-mounted power supply to a load using a cutoff switch such as a relay has been disclosed (JP 2010-239835A, for example). Specifically, a microcomputer provided in the protection device periodically obtains the current value in the electric wire from a current meter and calculates a value of the rise in the temperature of the electric wire using an expression of a relationship between the obtained current value, the heat generated by the electric wire, and the heat radiated by the electric wire. The temperature of the electric wire is calculated by adding a result of integrating the values of the rise in temperature calculated each time the current value is obtained to the ambient temperature at the start of energizing. When the calculated temperature of the electric wire reaches the limit temperature, the cutoff switch operates and cuts off the conductive path from the vehicle-mounted power supply to the load.
When carrying out maintenance or replacing parts in a vehicle, the vehicle-mounted power supply may be erroneously connected to the load in reverse. If the vehicle-mounted power supply is connected to the load in reverse, reverse overcurrent may flow in the electric wire and damage the electric wire, and thus it is desirable to quickly cut off such overcurrent.
An object of the present invention is to provide an electric wire protection device that can protect an electric wire by instantly cutting off a conductive path when a vehicle-mounted power supply is connected to a load in reverse.